Isn't the law is about first contact? his last contact with the ground before touching the ball for the first time was within the boundary
So it's out.
Having watched it again, I'm moving into the "It's woolly" territory. On some of the points:
1. He wasn't fully in control when he let the ball go, as if he was, he wouldn't have thrown it in a way which took it toward the boundary rope.
2. The ball appears to have never crossed the boundary, in which case the points being made relating to 19.4 are irrelevant.
On this basis, I change my verdict and go with "Out".
In addition, law 19.4 relates to the first fielder to touch the ball. Any subsequent fielder is not bound by this, so the question if law 19.4 was in play is, having knocked it back in, would he then be considered under the subsequent fielder rule or the first fielder rule...?